1. Field of the Invention
The present invention relates generally to an automotive hydraulic system for supplying a pressurized working fluid to a hydraulically operable automotive device, such as an automotive hydraulic suspension system. More specifically, the invention relates to a hydraulic pressure supply control system which is variable of pressure supply characteristics depending upon the vehicle driving condition. The present invention also relates to an actively controlled automotive suspension system employing a hydraulic pressure supply system for varying suspension characteristics according to the vehicle driving condition.
2. Description of the Background Art
In general, the conventional automotive hydraulic system includes an engine driven fluid pump as a fluid pressure source for supplying a pressurized working fluid to automotive devices, such as a hydraulic actuator, a hydraulic suspension damper and so forth. The engine driven fluid pump is generally driven by an output torque of the engine. The hydraulic system also includes a pressure relief valve for maintaining the working pressure in a pressure line below a predetermined pressure level.
In such engine driven fluid pump, the pump speed is naturally variable depending upon the engine revolution speed. This differentiates fluid pressure supply efficiency. In the hydraulically operable automotive devices, such as hydraulic suspension system, such variation of the fluid pressure in the hydraulic system will influence to performance or response characteristics of the automotive hydraulic devices. In order to maintain desired minimum performance of the automotive hydraulic device, the discharge ratio of the working fluid at the engine minimum revolution speed can be set at the rated flow. By this extra amount of working fluid to be relieved through a return line of the hydraulic system increases as increasing the engine revolution speed.
Here, assuming the set relief pressure of the pressure relief valve is Pr, the pump speed which corresponds to the engine speed is N, the discharge amount of the fluid pump per revolution is Q, power P to be consumed by the fluid pump can illustrated by: EQU P=(Pr+a).times.N.times.Q
where a is a constant. As will be seen herefrom, the consumed power P is proportional to the pump speed and thus increases according to increase of the engine speed. The consumed power P by the fluid pump serves as power lose of the engine to cause drop of fuel economy.
Furthermore, in the conventional system, since the conventional fluid pump maintains predetermined pump speed for maintaining the rated flow even in low engine speed condition, such as the engine idling condition while the vehicle is stopping. In case that the automotive hydraulic device is a hydraulic suspension system, such as an actively controlled suspension system, all of the pressure control valve is shut to maintain the fluid pressure constant so as not to cause height variation. This clearly wastes the engine output power.
Furthermore, even when the vehicle is running, the required fluid pressure may be variable depending upon the vehicle driving condition. Namely, when the vehicle is running in steady or stable condition in straight, required fluid pressure in the actively controlled suspension system will be substantially small since no substantial attitude change will occur. On the other hand, when the vehicle travels through a corner to cause vehicular rolling, higher fluid pressure has to be required for obtaining quick response to attitude change for quickly regulate the vehicular attitude. In order to satisfy the requirement for the higher fluid pressure at unstable vehicle driving condition, the fluid pressure during vehicle running condition has to be set at a level which is unnecessary for steady or stable state vehicle driving. This also serves to cause power lose and whereby to cause degradation of the fuel economy.
Furthermore, the power lose during the vehicle running condition may cause lowering of the vehicle acceleration characteristics and performance and degradation of the drivability.